Distributed Antenna System

ABSTRACT

A wireless communication network employs a distributed antenna system to provide radio coverage. The wireless communication network comprises a plurality of access points providing service in respective coverage areas. The access point within each coverage area connects to a plurality of antennas that are widely distributed within the coverage area. Radio resources at antennas within the overlapping region of two or more neighboring coverage areas are shared by the access points in the neighboring coverage areas according to a multiple access scheme. The sharing of radio resources within the overlapping region of two or more coverage areas allows the overlapping region to be enlarged, thereby providing more time to complete a handover.

TECHNICAL FIELD

The present invention relates generally to distributed antenna systemsfor mobile communications.

BACKGROUND

A distributed antenna system (DAS) or a distributed radio system (DRS)generally refers to a radio-access architecture comprising a largenumber of antennas (or radio heads) distributed widely across a largecoverage area and connected to a centralized access point (AP). Theradiation coverage of each antenna typically has a much smallerfootprint than that of a base-centrally-located antenna/base station ina conventional cellular system. The DAS architecture has two mainadvantages. First, it is possible to achieve high spatial re-usecapacity due to the small coverage area of each antenna. Second, thecentralized access point has complete control of all the radio resourcesused at each antenna and can therefore coordinate the transmission andreception of signals to minimize interference in an increased systemcapacity.

Typically, the antennas in a DAS are connected to the AP through opticalfibers. The AP may process the received (uplink) signals from multipledevices using appropriate combining techniques, such as maximum ratiocombing (MRC) or interference rejection combining (IRC). On thedownlink, the AP may transmit to multiple devices using zero forcing ordirty papercoding to suppress interference if the forward link channelis known. The AP may also use macro diversity techniques to directradiation to specific mobile devices if the channel is not known.

The capacity enhancements associated with DAS are well documented inisolated systems in which all antennas are connected to a single AP. Inreality, the coverage area of an AP is limited by factors such as fiberlength, propagation delay, and computing power. Therefore, a practicalDAS more than likely will have multiple APs within respective coverageareas and mobile devices will need to be handed over from one AP toanother in a manner similar to conventional cellular systems.

Handover may be approached in a DAS in a manner similar to aconventional cellular system. In a conventional system, the mobiledevice periodically makes signal strength quality measurements andreports the signal quality measurements to a radio network controllerthat coordinates handovers between a serving base station and a targetbase station. In general, a handover is triggered when the signalstrength from a target base station exceeds the signal strength from thecurrent serving base station by some predetermined amount.

The conventional method for handover can be problematic in a DAS. Aspreviously described, the radio coverage area of an antenna in a DAS ismuch smaller than that of a centrally-located antenna in a conventionalcellular system. Therefore, the overlap in coverage areas between twoAPs in a DAS is much smaller when compared to a conventional cellularsystem. For a mobile device traveling at moderate speed, the time inwhich to execute a handover will be much shorter than a conventionalcellular system. This brief period may not be sufficient to complete aseamless handover, which may lead to a brief interruption in service. Ina worst case, the handover may fail.

SUMMARY

The present invention relates to distributed antenna systems forwireless communication networks. A plurality of access points providesservice over respective coverage areas in the distributed antennasystem. The access point within each coverage area connects to aplurality of antennas that are widely distributed within the coveragearea. Radio resources at antennas within the overlapping region of twoor more neighboring coverage areas are shared by the access points inthe neighboring coverage areas according to a multiple access scheme.The sharing of radio resources within the overlapping region of two ormore coverage areas allows the overlapping region to be enlarged,thereby providing more time to complete a handover.

In a first exemplary embodiment, access points in two or moreneighboring coverage areas may connect to the same antenna in theoverlapping region of the coverage areas. In this case, the radioresources at the shared antenna are shared between the access points.For example, the access points in the neighboring coverage areas may beassigned different frequencies, different time slots, and/or differentcodes to use for transmission and reception of signals.

In another exemplary embodiment, the access points in the neighboringcoverage areas each connect to a respective subset of antennas in theoverlapping region. Radio resources are shared by allocating radioresources between antennas connecting to different access points.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an exemplary wireless communication network using adistributed antenna system.

FIG. 2 illustrates a method of handing over a mobile device in awireless communication network using a distributed antenna system.

FIG. 3 illustrates an exemplary access point for the wirelesscommunication network of FIG. 1.

DETAILED DESCRIPTION

Referring now to the drawings, FIG. 1 illustrates an exemplary wirelesscommunication network indicated generally by the numeral 10. Thewireless communication network 10 comprises a plurality of access points100 that provide radio access to mobile devices 200 within theirrespective coverage areas 12. In FIG. 1, two axis points 100 are shownand are denominated as AP1 and AP2, respectively. Those skilled in theart will appreciate, however, that a typical network 10 may include manyaccess points 100 and corresponding coverage areas 12. A radio networkcontroller (RNC) 300 connects to each access point 12. One function ofthe radio network controller 300 is to manage handover of mobile devices200 from one access point 100 to another as hereinafter described.

In a conventional cellular system, each access point 100 typicallyconnects to a single antenna located at the center of each coveragearea. According to the present invention, each coverage area uses adistributed antenna system (DAS) rather than a single, centrally locatedantenna. The access point 100 in each coverage area connects to aplurality of antennas 14 that are widely distributed over each coveragearea 12. The radiation coverage of each antenna 14 is typically muchsmaller than a base station antenna in a conventional cellular system.However, the antennas 14 in each coverage area 12 collectively provideradiation coverage throughout the entire coverage area 12.

The DAS architecture has two main advantages. First, it is possible toachieve high spatial re-use capacity due to the small coverage area ofeach antenna 14. Second, the centralized access point 100 has completecontrol of its allocated radio resources at each antenna 14 and cantherefore coordinate the transmission and reception of signals tominimize interference in an increased system capacity. As will bedescribed in greater detail below, the radio resources at some antennas14 may be shared with other access points 100.

As shown in FIG. 1, the coverage areas of AP1 and AP2 in the presentinvention overlap to include one or more antennas 14. The overlappingregion of the neighboring access points 100 is designated by referencenumeral 16. In various embodiments, the transmit antennas 14 in theoverlapping region 16 may connect to one or both of the neighboringaccess points 100. In either case, the radio resources at the transmitantennas 14 in the overlapping region 16 are shared between theneighboring access points 100 according to a multiple access scheme(e.g., TDM, FDM, CDM, OFDM, etc.). By allowing neighboring access points100 to share radio resources within the overlapping region 16, it ispossible to enlarge the overlapping region 16 to provide more time toexecute a handover when a mobile device 200 is moving at moderatespeeds.

In a first exemplary embodiment, the antennas 14 in the overlappingregion 16 of the neighboring coverage areas 12 are connected tocorresponding access points 100 in each of the overlapping coverageareas 12. That is, each antenna 14 connects to the access point 100 foreach of the neighboring coverage areas 12. In the embodiment shown inFIG. 1, there are only two overlapping coverage areas 12. Thus, eachantenna 14 in the overlapping region 16 connects to both AP1 and AP2.However, those skilled in the art will appreciate that some antennas 14could be located in a region where three or more coverage areas 12overlap. In this embodiment, the radio resources at each antenna 14 areallocated to respective ones of the access points 100. The radioresources at each antenna 14 may be fully utilized; however, each accesspoint 100 in the neighboring coverage areas 12 will have control of onlya portion of the radio resources at each antenna 14.

Any known multiple access scheme may be employed to divide the radioresources between neighboring access points 100. For example, theneighboring access points 100 may be assigned different frequencies touse at each antenna 14. Alternatively, neighboring access points 100 maybe assigned different time periods or different codes to use at eachantenna 14. Radio resources could also be shared using orthogonalfrequency division multiplexing (OFDM). Those skilled in the art willappreciate that other multiplexing techniques also exist and that theenumeration of multiplexing techniques herein is not intended to limitthe scope of the claims to the enumerated techniques.

In another exemplary embodiment, each of the transmit antennas 14 in theoverlapping region 16 between neighboring coverage areas 12 connect to aselected one of the access points 100. Thus, each access point 100connects to a subset of antennas 14 in the overlapping region 16 betweencoverage areas 12. The transmit antennas 14 in the overlapping region 16are densely packed and intermingled so that each access point 100 mayprovide coverage throughout the overlapping region 16 with its subset ofantennas 14. Again, because multiple access points 100 compete toprovide coverage in the overlapping region 16, a multiple access schemeis used to divide the radio resources between the access points 100. Inthis embodiment, each subset of antennas 14 uses its allocated portionof radio resources in the overlapping region 16. The subsets of antennas14 combined use all of the radio resources in the overlapping region 16.

The allocation of radio resources at antennas 14 within the overlappingregion 16 between two neighboring coverage areas 12 may be preconfiguredby the service providers. Any orthogonal multiple access technique maybe used, such as TDM, FDM, CDM, and OFDM. The allocation of radioresources does not have to be equal. An access point 100 having greatertraffic may be allocated more radio resources than a lightly loadedneighboring access point 100.

The allocation of radio resources within the overlapping region 16 doesnot have to be fixed. In some embodiments, the allocation of the radioresources in the overlapping region 16 may slowly adapt to changingenvironment and traffic conditions. Signaling between neighboring accesspoints 100 may be used to reallocate radio resources from one accesspoint 100 to another access point 100 depending, for example, on theload at each access point 100. For example, if AP1 in FIG. 1 is heavilyloaded, it may request additional radio resources from AP2. If the radioresources for AP2 are not fully utilized, AP2 may allow AP1 to use thoseresources for an agreed period of time. For example, AP2 may allow AP1to use certain frequencies, time slots, or codes for the agreed periodof time.

Another way to balance load between neighboring access points 100 is toreassign mobile devices 200 within the overlapping region 16 even thoughthe mobile devices 200 are not in need of handover. This techniqueapplies primarily to mobile devices 200 that are moving slowly and maytherefore be served by any one of the neighboring access points 100. Asan example, when AP1 is heavily loaded, it may request AP2 to accepthandover of a mobile device 200 within the overlapping region 16 inorder to reduce its load.

As in a conventional cellular system, mobile devices 200 will be in needof handover as the mobile devices 200 move from one coverage area into adifferent coverage area. Preferably, handover is not initiated until themobile device 200 reaches a handover boundary 20, which is beyond themidpoint of the overlapping region 16 as shown in FIG. 2. The handovermay be triggered by either the mobile device 200 or by a network device,such as the RNC 300. For example, the mobile device 200 may measure thetotal power received from antennas 14 on both sides of the handoverboundary 20. A handover is requested by the mobile device 200 when thereceived signal power from antennas 14 on the target side of thehandover boundary 20 exceeds the received signal power from antennas 14on the serving side of the handover boundary 20. Alternatively, thenetwork device may measure the total power received from the mobiledevice 200 by antennas 14 on both sides of the handover boundary 20 andinitiate a handover accordingly. The latter approach may be morepreferable since the access points 100 will know the location of thehandover boundaries 20.

In a preferred embodiment, a first handover boundary 20 is used formobile devices 200 moving in a first direction, and a second handoverboundary 20 is used for mobile devices 200 moving in the oppositedirection, as shown in FIG. 2. It will be appreciated that the first andsecond handover boundaries 20 do not coincide with the boundaries of theoverlapping region 16. The first and second handover boundaries 20 aregeographically separated to avoid the ping-ponging effect. Thus, when amobile device 200 moves from a first coverage area towards a secondcoverage area, handover is determined by a first handover boundary 20.If the mobile device 200 subsequently reverses direction, handover willbe determined by a second handover boundary 20. Thus, consecutivehandovers will not occur in rapid succession, which could be the case ifa single handover boundary 20 was used.

FIG. 3 illustrates an exemplary access point 100. Access point 100comprises a control processor 102, baseband signal processor 104, andswitching circuits 106. The control processor 102 controls overalloperation of the access point 100 according to any known communicationstandard. The baseband signal processor 104 processes signalstransmitted to and received by the access point 100. Exemplaryprocessing tasks performed by the baseband signal processor 104 comprisemodulation/demodulation, coding/decoding, interleaving/de-interleaving,spreading/de-spreading, etc. Switching circuits 106 connect the accesspoint 100 to the antennas 14 within its coverage area.

The present invention may, of course, be carried out in other specificways than those herein set forth without departing from the scope andessential characteristics of the invention. The present embodiments are,therefore, to be considered in all respects as illustrative and notrestrictive, and all changes coming within the meaning and equivalencyrange of the appended claims are intended to be embraced therein.

1-23. (canceled)
 24. A wireless communication network comprising: aplurality of access points providing service to mobile devices overrespective coverage areas; and within each coverage area, a plurality ofantennas distributed within the coverage area and connected to saidaccess point; wherein access points in neighboring coverage areas thatoverlap share, according to a multiple access scheme, radio resources atantennas within an overlapping region of said neighboring coverageareas.
 25. A method in a wireless communication network, which networkcomprises a plurality of access points providing service to mobiledevices over respective coverage areas, wherein each access pointconnects to a plurality of antennas distributed over its coverage area,said method comprising: overlapping coverage areas of neighboring accesspoints; and sharing radio resources at antennas in an overlapping regionof said coverage areas between access points in said neighboringcoverage areas according to a multiple access scheme.
 26. The method ofclaim 25, further comprising: connecting a first subset of antennas insaid overlapping region to an access point in a first one of said accesspoints in said neighboring coverage areas; and connecting a secondsubset of antennas in said overlapping region to a second one of saidaccess points in said neighboring coverage areas.
 27. The method ofclaim 25, further comprising connecting one or more of said antennas insaid overlapping region of neighboring coverage areas to two or moreaccess points in said neighboring coverage areas.
 28. The method ofclaim 25, further comprising pre-configuring access points in saidneighboring coverage areas to use certain shared radio resources atantennas within said overlapping region.
 29. The method of claim 25further comprising reallocating shared radio resources at antennaswithin said overlapping region between neighboring base stationsresponsive to changing conditions.
 30. The method of claim 29, whereinsaid access points negotiate directly to reallocate said shared radioresources.
 31. The method of claim 25, further comprising: setting afirst handover boundary in said overlapping region for mobile devicesmoving from a first coverage area toward a second coverage area; andsetting a second handover boundary in said overlapping region spacedfrom said first handover boundary for mobile devices moving from saidsecond coverage area toward said first coverage area.
 32. The method ofclaim 31, further comprising initiating a handover of a mobile devicesin said overlapping region based on the strength of received signalsfrom said mobile device at antennas on a target side and a serving sideof a handover boundary.
 33. The method of claim 31, further comprisingsaid mobile device in said overlapping region initiating a handoverbased on the strength of received signals at said mobile devices fromsaid antennas on a target side and a serving side of a handoverboundary.
 34. An access point for a wireless communication networkcomprising a plurality of access points providing service to mobiledevices over respective coverage areas, wherein the access point in eachcoverage area connects to a plurality of antenna distributed within thecoverage area, and wherein the coverage area of said access pointoverlaps with the coverage area of a neighboring access point, whereinsaid access point is configured to share radio resources with aneighboring access point at antennas within an overlapping region ofsaid neighboring coverage areas, according to a multiple access scheme.35. The access point of claim 34, wherein said access point connects toa first subset of antennas in said overlapping region, and wherein saidneighboring access point connects to a second subset of antennas in saidoverlapping region.
 36. The access point of claim 34, wherein saidaccess point shares at least one antenna in said overlapping region witha neighboring access point.
 37. The access point of claim 34, whereinsaid access point is pre-configured to use certain shared radioresources at antennas within said overlapping region.
 38. The accesspoint of claim 37, wherein said access point is configured to negotiatedirectly with a neighboring access point to reallocate the shared radioresources in said overlapping region.